THE SYMPATHETIC NERVOUS SYSTEM IN MAMMALS. 
183 
(2) My own Investigations reveal definitely tlie earliest appearance, and the 
complete history of the formation of the sympathetic system in Itodents (a compara¬ 
tively primitive group of Mammals) ; and distinctly indicate the existence of a 
considerable period during which the sympathetic cord is entirely independent of the 
cerebro-spinal system. 
(3) Investigators who have assigned an epiblastic origin to the sympatlietic system 
differ materially as to the actual mode of development. As already stated (p. 1G2), 
Balfour regards the sympathetic ganglia (in Elasmobranchs) as outgrowths from 
the spinal nerves ; while Onodi (working at the same group) maintains that they are 
direct proliferations from the spinal ganglia. Moreover, these authors have left the 
history of the development of the system in an incomplete state. The earliest steps 
in the process have not been described for Mammals ; and the ultimate and peripheral 
branches and ganglia have not been traced back to the epiblast. The fragmentary 
nature of the observations, and the divergence of opinion regarding them, show that 
further investigations are necessary before the origin of the sympathetic system as an 
outgrowth from the cerebro-spinal system can be accepted as proved even for any 
group of vertebrates, still less for all. 
(4) Besides the embryological facts recorded in the preceding pages, it may be 
further pointed out that there are strong grounds, histological and ])hysiological, for 
drawing a sharp distinction between the sympathetic and the cerebro-spinal nervous 
systems. 
The cells comprising the sympathetic ganglia differ from those of the spinal ganglia 
in histological character, the latter being rounded, the former angular. In birds, 
Ramage (25) describes the difference as being very evident, “ que les cellules des 
ganglions sympathicjues etaient nettement multipolaires, et qu’il etait facile de les 
distinguer de cedes du ganglion spinal, qui sont spheriques et unipolaires.” The 
fibres of the sympathetic system are essentially different from those of the cerebro¬ 
spinal system, being non-meclullated branching fibres. 
Physiologically also, the sympathetic is clearly distinct from the cerebro-spinal 
system. Its functions appear to be to direct and regulate the distribution of certain 
nerves which emanate from the cerebro-spinal axis. The ganglia are not stated by 
physiologists to be either automatic or reflex, but only trophic, from which we may 
infer that they have a ‘Wital influence ” over the parts within their jurisdiction. The 
ganglia apparently neither augment, diminish, nor alter the character of the nerve 
impulses which permeate them. 
From these considerations it may be regarded as very possible that under the term 
“ sympathetic nervous system ” are included two structures, entirely independent in 
nature, origin, and function, the sympatlietic system, and the nervous system proper. 
Such a view would be in entire harmony with tlie mode of development described 
above, and would I’ender more intelligible than has hitherto been possible certain 
developmental peculiarities, such as the conversion of part of the sympathetic system 
into the suprarenal bodies. 
